Methods of and means for sealing the terminal opening of an electric heater



March 3, 1959 E A. c. BOGGS 2,876,322 I METHODS OF AND MEANS FOR SEALING THE! TERMINAL OPENING OF AN ELECTRIC HEATER Filed July 5. 1955 INVENTOR. ALBEN C. Boscss United States Patent METHODS OF AND MEANS FOR SEALING THE TERMINAL OPENING OF AN ELEC- TRIC HEATER Application July 3, 1953, Serial No. 365,999

Claims. (Cl. 201-67) My invention relates'to electric heaters, more particularly to methods of and means for sealing the terminal opening of electric heaters, and the principal object of my invention is to produce new and improved methods of and means for sealing such opening.

A terminal seal for an electric heater must meet many exacting requirements and failure to meet any one of such requirements often results in failure of the heater.

My invention provides a seal for effectively sealing the terminal opening of an electric heater and one which may be cast or molded directly into the pocket formed at the sheath opening. Such seal will set in a comparatively short time and is characterized by high adhesion to surfaces with which it is in contact, low shrinkage, high mechanical strength and high moisture resistance. In the drawing accompanying this specification and forming a part of this application, there are shown, for purpose of illustration, several forms in which my improved seal may be used, and in the drawing:

Figure l is a fragmentary longitudinal sectional view through a conventional sheathed-type tubular electric heater, showing use of my invention,

Figure 2 is a fragmentary longitudinal sectional view through a similar electric heater, showing a modified form of seal, I

Figure 3 is an end elevational view of another form of electric heater, and

Figure 4 is a fragmentary sectional view corresponding generally to the line 4-4 of Figure 3.

Referring particularly to Figure l of the drawing, the 2 electric heater therein disclosed comprises a tubular sheath 10 which is usually of metal and an electric resistance element (not shown) within the sheath. A terminal pin 11 is electrically connected to the resistance element and extends outwardly of the opening of the sheath for proper electrical connection to a source of electrical energy.

Suitable electric-insulating heat-conducting material 12 is disposed within the sheath and serves to both electrically insulate the resistance element and the terminal pin from the sheath and also conduct heat from the resistance element to the sheath. Such material may take the form of suitable powdered magnesium oxide which has been highly compacted within the sheath. The material 12 stops short of the sheath opening to define a pocket 14 with the adjacent inner surface of the sheath.

In accordance with my invention, an epoxy-type resin 15 is disposed within the pocket to seal the terminal opening of the sheath. Resins of this type are presently reaction products of bisphenol with epichlorohydrin and are characterized by high. adhesion, low shrinkage, high mechanical strength and high moisture resistance.

Epoxy-type resins are usually supplied as two-part materials consisting of resin and hardener, or activator, the resin itself being a linear polymer and the hardener or activator being capable of producing cross-linkage so that once the material has set it resists remelting.

The resins are commercially available in hot-pouring ice or cold pouring types. One form of the hot-pouring type presently comes in solid form together with a solid hardener. These materials are heated until liquefied and may then be poured into the pocket 14 which for all intents and purposes acts as a mold cavity. The liquefied material presently sets and cures at about 250 degrees F. in about four to eight hours.

One form of cold pouring type of resin presently available comes in liquid form with a choice of two liquid hardeners and may be cured in about 3 to 6 hours at room temperature. I

Either type resin may be used for most applications depending largely upon production facilities available. Since the cold pouring resin requires less equipment it may be used in many applications although the hot pouring resin may be more economical in large scale operations.

In either event, the epoxy-type resin forms a substan-' tial seal for the terminal opening of the heating element since it adheres with great strength to the adjacent surfaces of the sheath and terminal pin and forms a moisture resistant plug of high mechanical strength and free of internal stresses which may be caused by shrinkage.

It will be understood that the epoxy-type resin, when ready for use, is pourable and has a consistency something like honey. Preferably, the resin is poured into the opening 14 immediately after the heating element comes from the drying and baking operation wherein any moisture in the element is driven out. The heat retained by the element following the foregoing operation is useful in that it makes the liquid resin still more pourable and tends to maintain its fluidity a sufficient time so that it enters all cracks or openings and thus provides a more efiicient seal. Further, the heat of the element thereafter assists in setting the resin to its final condition to form a hard sealing body.

After the resin has set, the element may be ready for commercial use. However, in some instances it is preferred to electrically and mechanically connect a rubber covered lead to the terminal pin and thereafter mold a rubber sleeve 16 about the end of the sheath and over and about the adjacent end of the rubber covered lead, as shown in Figure 1.

In Figure 2, a hard dielectric bushing 17, which may be formed of lava, is pressed into the resin 15 before the latter has set.

adjacent surfaces of the sheath and the terminal pin. A shoulder 20 is formed on the bushing 17 and this shoulder is pressed against the peripheral lip of the sheath by means of a nut 21 which is threaded onto a threaded portion of the terminal pin. Mica washers 22 may be interposed between the nut 21 and the bushing 17, if

desire-d. It will be noted that the opening through the bushing 17 is slightly larger than the adjacent external diameter of the terminal pin 11 and that resin is extruded in the space thus provided, as shown at 23, to enhance sealing operations.

My invention may be used as a seal for practically all types of heating elements and Figures 1 and 2 of the drawing show the invention applied to tubular elements wherein a terminal extends outwardly from each The bushing has annular tapered sur-' faces 18 and 19 which tend to wedge the resin toward both terminal pins 25--25 extend outwardly from the opposite end 26.

The end 26 of the sheath 24 is provided with enlargements 27--27 in line with respective terminals, and such enlargements may be formed by bulging the sheath by means of a press operation.

As before, the terminals 25 are connected to a resistance element (not shown) and in the particular construction disclosed the terminals 2525 are connected to opposite ends of the resistance element. The usual compacted refractory material 28 insulates the resistance element and the terminals 25 from the sheath 24, and such material stops short of the end of the sheath so that a pocket 29 is formed which extends entirely across the sheath end.

The epoxy-type resin 30 is then poured into the pocket 29 to fill the enlargements 27 and the remaining portions of the pocket and is permitted to set to form the seal. If desired, mica washers 31 may be disposed over each terminal pin 25 and a nut 32 threaded onto a threaded portion of the terminal pin to clamp the washers against the end of the enlargements 27.

In view of the foregoing it will be apparent to those skilled in the art that I have accomplished at least the principal object of my invention and it will also be ap- 1. An electric heater, comprising a tubular sheath having an opening therein, a resistance conductor disposed within said sheath and having a terminal extending through the opening in said sheath, electric-insulating heat-conducting material within said sheath for electrically insulating said resistance conductor and said terminal from said sheath, said material stopping short of the end of said sheath at said opening and defining a pocket therewith, and a seal within said pocket and around the adjacent portion of said terminal for sealing the opening of said sheath, said seal being a reaction product of bisphenol with epichlorohydrin.

2. An electric heating element comprising a tubular sheath having an opening therein, a resistance conductor disposed within said sheath and having a terminal extending through the opening in said sheath, electricinsulating heat-conducting material within said sheath for electrically insulating said resistance conductor from said sheath; said material stopping short of filling said sheath opening to form the bottom of a pocket which also includes the interior wall of said sheath at its opening, and an insulating seal within said pocket closely adhering to the adjoining surfaces of said sheath and said terminal and comprising an in situ cured epoxy 4 resin which was introduced into said pocket in liquid form, said epoxy resin being a reaction product of hisphenol with epichlorohydrin.

3. An electric heating element, comprising a tubular sheath having an opening therein, a resistance conductor disposed within said sheath and having a terminal extending through the opening in said sheath, electricinsulating heat-conducting material within said sheath for electrically insulating said resistance conductor from said sheath, said material stopping short of filling said sheath opening to form the bottom of a pocket which also includes the interior wall of said sheath opening, and an insulating seal within said pocket closely adhering to the adjoining surfaces of said sheath and said terminal and comprising an epoxy resin which is disposed within said pocket.

4. The method of sealing, and providing a rigid terminal support at, the terminal end of an electric heater wherein a resistor enclosed in an open end tubular sheath is embedded in insulation terminating at a distance from the end of the sheath to leave the sheath with an open end recess forming a mold cavity through which a terminal portion of the resistor projects, which comprises introducing a fluid epoxy resin into said mold cavity to fill all voids between and adhere to opposing surfaces of said cavity and said terminal portion and permitting said resin to harden to provide a sealing body having high mechanical strength to rigidly support said terminal portion and also having high moisture resistance to restrict moisture from entering said tubular sheath through the sheath end.

5. The method of sealing, and providing a rigid terminal support at, the terminal end of an electric heater wherein a resistor enclosed in an open end tubular sheath is embedded in insulation terminating at a distance from the end of the sheath to leave the sheath with an open end recess forming a mold cavity through which a terminal portion of the resistor .projects, which comprises subjecting the heater to a baking operation to remove moisture from the interior of the sheath and introducing fluid epoxy resin into said mold cavity while the heater is still hot from the baking operation so that it will flow to fill all voids between and adhere to opposing surfaces of said cavity and said terminal portion, and permitting said resin to harden to provide a sealing body having high mechanical strength to rigidly support said terminal portion and also having high moisture resistance to restrict moisture from entering said tubular sheath through the sheath end.

References Cited in the file of this patent UNITED STATES PATENTS 2,489,998 Charbonneau et al. Nov. 29, 1949 2,527,890 Pouchnik et al. Oct. 31, 1950 2,538,808 Swiss Jan. 23, 1951 2,622,133 Dorst Dec. 16, 1952 2,659,795 Boggs Nov. 17, 1953 

